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5), indicating a high degree of conformational flexibility of the DAD chains.
All oligomers on the surface reveal high structural regularity.
In particular, the combination of alternating oligothiophene donors and benzobis(1,2,5-thiadiazole) acceptor units, which in addition facilitate the formation of quinoid structures, has proven to lower the HOMO–LUMO gaps far below 1 e V (ref. Very recently, Fasel and co-workers have used on-surface synthesis to prepare a molecular chain with a random mixture of undoped and doped graphene nanoribbon sections, thus creating characteristic electronic transitions between them in analogy to p–n junctions.
However, to obtain a well-defined alternation of donor and acceptor units, on-surface polymerization involving step growth via dehalogenative Ullmann-type coupling reactions requires the integration of both donor and acceptor units into one symmetrical monomer structure carrying two reactive halide termini and hence we prepared bis(5-bromo-2-thienyl)-benzobis(1,2,5-thiadiazole) as the Br-DAD-Br monomer (Fig. The abbreviation DAD stems from the alternation of donor (D) and acceptor (A) groups, namely an (acceptor) central benzobis(thiadiazole) unit carrying two lateral (donor) thiophene rings.(a) Chemical structure of the Br-DAD-Br molecule and (b,c) molecules on an Au(111) surface before polymerization.
The latter method, pulling a single molecular wire with the tip apex of a scanning tunnelling microscope (STM) at low temperatures, is slower and therefore provides much less data in a reasonable time frame.Their characteristic shape is in agreement with our molecular design and proves the successful polymerization process, leading to the desired molecular wires composed of benzobis(1,2,5-thiadiazole) moieties linked via flexible bithiophene units.In order to gain insight into electronic delocalization along the molecular wires, the intramolecular electronic structure of single chains was spatially resolved using low-temperature scanning tunnelling spectroscopy (STS).Back in 1974, Aviram and Ratner have theoretically proposed to study long-range electron transfer processes through a single molecule composed of an electron-donating subunit covalently connected via a bridge to an electron-accepting subunit, thereby conceiving a molecular rectifier.However, realizing their vision and exploring charge transfer at the level of a single molecule is a challenging task that continues to attract considerable attention, both from theory.
By placing the STM tip at different positions over the molecule (and over the pure gold surface as a reference to eliminate contributions from the tip apex and the metal surface states), the d I/d V spectrum at the acceptor location (that is, the benzobis(1,2,5-thiadiazole) group) reveals the presence of two clear features, one at low energy (A3 e V), respectively.